The most appropriate piece of evidence to support the author's claim would be:
D. Many studies have shown that schools who offer healthy alternatives to traditional menu items have a reduced number of obese students.
This option directly relates to the argument that schools should regulate what students eat and drink on campus by providing evidence that healthier food options can lead to better health outcomes, specifically in reducing obesity rates among students. This strengthens the author's argument by linking healthy dietary choices to tangible benefits.